1. Technical Field
The present invention relates to a method of forming a bonded structure, as well as a circuit board, an electro-optic device and an electronic device.
2. Related Art
One conventional coupling method for bonding a driver IC on a substrate of a display device is a known as chip-on-glass (COG) coupling. The COG coupling employs a method wherein, for example, a driver IC is bonded by forming an Au-plated bump on a driver IC and then electrically coupling the bump formed on the driver IC with an electrode terminal formed on a substrate of a display device using a conductive bonding material such as anisotropic conductive film (ACF) and anisotropic conductive paste (ACP) (see Japanese Unexamined Patent Publication Nos. 2-272737 and 3-96921, for example).
However, with the miniaturization (pitch narrowing) of the electrodes of a driver IC, the size of conductive particles contained in the above conductive bonding material such as ACF and ACP is becoming close to the size of the gap between the electrodes described above. This sometimes causes a short circuit between the electrodes of the driver IC. Therefore, it has become difficult to mount a driver IC using a conductive bonding material.
Instead, with the pitch narrowing of electrodes of a driver IC, non-conductive films (NCF), that do not contain conductive particles have been widely used as a substitute for conductive bonding materials such as ACP, ACF and the like. Further, as the bump formed on a driver IC, a resin having a core of polyimide having an excellent heat resistance is used (see Japanese Unexamined Patent Publication No. 6-302606, for example).
However, since a resin protrusion made of polyimide has a high elastic modulus lower than a driver IC bonding temperature (high temperature), the resin does not make a transformation at the time of bonding, which has caused a problem of reduction in coupling reliability. More specifically, in COG bonding, an electrode of a driver IC and an electrode terminal on a substrate of a display device are electrically coupled with each other using NCP. Further, the coupling is fixed and retained. However, as for a resin protrusion made of polyimide configuring an electrode of a driver IC, the resin is hardened and contracted by hot pressing, making the periphery of the resin protrusion top raised. Thus, the center of the resin protrusion top is dented (e.g., recessed) compared to the periphery. Therefore, in COG bonding, NCF stays in the recess on the resin protrusion top, which may cause poor conduction in such a region and eventually a reduction in the coupling reliability of the entire device due to the poor conduction.
Hence, as a solution to the above problem, it is considered that, with the use of a silicon-based resin having a low elastic modulus as a bump, the rise of the periphery of the resin protrusion top due to the transformation of the resin at the time of bonding can be avoided and the coupling reliability between an external electrode of a driver IC and an electrode terminal of a display device can be improved. However, there is another problem in that the typical material type of resin having a lower elastic modulus is generally limited to, for example, silicon.
The present invention has been developed taking the above problems into consideration and aims to provide a method of forming a bonded structure that can improve the coupling reliability between a resin protrusion of a driver IC and an electrode terminal formed on a substrate of a display device, as well as a circuit board, an electro-optic device, and an electronic device.